1. Field of the Invention
The present invention relates to an improvement in the uniformity of one or more coatings of solid or semisolid materials applied on particulate substrates; more particularly it relates to highly uniform and effective coatings which may be either of the same, or of different composition, as the particulate substrate, as well as layers, each of a different composition, which may be applied consecutively and uniformly in the desired proportions; and still more particularly it relates to coating media that are normally solids or semisolids at ambient temperatures but that can be reduced to a sprayable liquid state by melting or by dissolution or suspension in an appropriate solvent and that can be quickly returned to the solid or semisolid state by cooling below the fusion temperature or by rapid evaporation of the solvent, it being understood, of course, that such cooling may be achieved by passing a cooling gas, such as ambient air, through the unit in contact with the product, or more simply by maintaining the particulate feed temperature at a sufficiently low temperature that the resulting product emerges at a temperature below the fusion temperature of the coating medium, and that evaporation of a solvent may be achieved by passing heated gases through the unit in contact with the product.
The invention is particularly well-suited to the application of uniform coatings of fusible compounds, elements, or mixtures on particulate, water-soluble fertilizer compounds for the purpose of producing controlled-release fertilizers, but is not limited to processing materials of this type.
The application of coatings, or encapsulations, on particulate substrate by the spraying of melts, solutions or suspensions onto moving beds of the particles in a wide variety of types of equipment is well known in the prior art. The application of coatings of the same material as the particulate substrate is commonly referred to as granulation and is widely used in the fertilizer industry, among others. Coating with materials different than the particulate substrate is more aptly termed coating, or encapsulation, and is also practiced in the fertilizer industry to some extent, but more widely in the production of pharmaceuticals such as pills or tablets and food products such as candies or nuts. Coating per se is not practiced widely in fertilizer industry at this time because of the difficulty in obtaining adequately uniform coatings at high-tonnage capacities with low-cost coating media.
The most demanding requirements for uniformity of coating, both from the standpoint of completeness of the coating around each individual particle and of maintaining the same relative proportions of the coating medium to the substrate throughout the mass of particles occur in the production of certain pharmaceuticals. In this field, the coating may be either an active ingredient which must be proportioned correctly on each substrate pill, or it may be a permeable, inert coating designed to release the medically active component slowly over a period of hours. In the latter case, nonuniform coatings with imperfections may release dangerous amounts of the substrate drug too rapidly. On the other hand, the high price and low tonnage production of drugs justifies the use of high-cost coating media in small sophisticated expensive coating units that are usually operated in relatively small batches.
It is in the fertilizer industry that the overall requirements for an economical continuous, high-capacity coating unit, capable of applying a highly uniform coating, are most difficult to meet. Although it is apparent to those versed in the art that the instant invention as disclosed in the following sections is readily adaptable on a small scale to the requirements of the pharmaceutical industry, or to other similar applications; they will also recognize that the instant invention is particularly adaptable to the requirements in the fertilizer industry for simple, low-cost equipment and for high-tonnage production capacity. Therefore, the discussions and disclosures in the following sections describe the prior art and the application of the instant invention to coated fertilizer production, both for the purpose of granulation, i.e., gradual increase of undersized nuclei to product size by successive layering, or coating, of the same or dissimilar fertilizer compounds; and for the purpose of applying permeable or semipermeable coatings of natural or synthetic polymeric compounds, oils, waxes, asphaltic mixtures, sulfur, and combinations of these materials to product size particulate fertilizer substrate in order to provide a cnrolled-release fertilizer, a product of proven desirable characteristics to those versed in the art, provided that it can be produced in simple equipment at a reasonable cost.
2. Description of the Prior Art
U.S. Pat. Nos. 3,117,020, T. Fabris et al (Jan. 7, 1964), 3,165,395, I. W. McCamy et al (Jan. 12, 1965), and 3,211,522, R. A. Shurter et al (Oct. 12, 1965) disclose the granulation of undersized fertilizer compound nuclei (recycled fines from product screened) by spraying a hot, concentrated aqueous solution or a nearly anhydrous melt of the compound onto a rolling bed of the nuclei in an inclined, rotating pan. Although there is some coating action, most of the granulation, or increase to product size, is accomplished by agglomeration, or sticking together, of a number of the nuclei by the solidifying melt or solution. The granules are relatively rough and irregular, compared with layer granulation (see later section), and coating per se is minimal. The unit has little application as a true coating unit because of its natural classification action.
In U.S. Pat. No. 2,815,376, R. E. Knowlton et al (Dec. 3, 1957), there is disclosed the granulation of fertilizer compounds by the spraying, or simple mixing, of the undersized fertilizer compound nuclei with a hot solution or melt of the compound in a paddle mixer (blunger or pugmill). Again, granulation is by agglomeration of several nuclei into a single, larger particle by the solidifying melt. Little coating action is achieved and the particles are rough and irregular.
It has also been shown that granulation, predominantly by a true coating, or layering, action can be accomplished by spraying the hot solution or melt of a fertilizer compound into a fluidized bed of undersized compound nuclei. In U.S. Pat. No. 2,600,253, W. A. Lutz (June 10, 1952) ammonium nitrate or ammonium sulfate fertilizers are produced by reacting ammonia and nitric acid or sulfuric acid in a fluidized bed of undersized ammonium nitrate or ammonium sulfate particles. In other applications, principally in Europe, a hot melt or concentrated solution of the compound is sprayed into the fluidizing gas (hot air) at the bottom of the fluidized bed. The fluidized bed does achieve the truly random motion of the substrate particles, and therefore a homogeneous mass with respect to particle size, that is so necessary in uniformly coating particles of varying sizes. However, the fluidization process is inherently costly; it requires close control, and it does not permit visual examination of the sprays or product in the coating section.
In the fertilizer industry, granulation, i.e., increase in particle size from undersize (recycled fines) to product size by coating, and coating for the purpose of imparting special characteristics to the fertilizer substrate, such as controlled release or anticaking properties, is most widely practiced in essentially horizontal rotary drums either with or without internal lifting vanes or flights. U.S. Pat. No. 3,398,191, J. B. Thompson et al (Aug. 20, 1968), discloses the granulation of urea or ammonium nitrate by spraying an essentially anhydrous melt (98-99.5 percent) of the coating compound from multiple spray heads spaced at intervals along the entire length of the coating section onto a rolling bed and into a showering curtain of undersized nuclei (recycled fines) maintained in motion by the rotation of a slightly inclined (from horizontal) rotating drum equipped with longitudinal lifting vanes, or flights, specially designed to form continuous longitudinal curtains of falling particles that move in succession across the entire cross sectional area of the contact, or coating, zone of the drum in a manner familiar to those versed in the art of horizontal rotary drum coolers and dryers. Transverse retaining rings, or dams, at the feed end and discharge end of the coating section maintain an adequate depth of bed. Cooling air (ambient temperature) is drawn countercurrently through the showering curtains of falling particles to cool and solidify the layers of melt on the nuclei. The drum is extended beyond the contact zone to a cooling zone, equipped with the lifting flights, but without spray heads, with which to further cool the product with the countercurrent flow of air.
Other essentially identical examples of this form of prior art featuring the falling curtain across the full cross sectional area of the rotary drum are disclosed in U.S. Pat. Nos. 3,092,489, Smith (June 4, 1963), 3,227,789, Tytus et al (Jan. 4, 1966), and 3,232,703, Thompson et al. (Feb. 1, 1966).
The falling curtains of particles across the full cross sectional area of the drum, as described in the above prior art, approaches the degree of random motion of substrate, and therefore of homogeneity with respect to particle size, that is so important to the uniform coating of a mass of particles of different sizes, but it is now believed that the arrangement of spary heads within the shower of falling particles, and therefore in actual contact with many of the falling particles as a result of the falling curtains across the entire cross sectional area of the drum, has certain serious disadvantages. Among the most serious of these disadvantages are (1) the lack of control of the spray distance, i.e., the distance that the individual droplets of atomized liquid spray travels before impinging upon the moving substrate particles; some of the particles fall on each spray head, some immediately in front of it, some fall at the optimum distance, and some fall well beyond the optimum distance but are still contacted by the spray. This leads to agglomeration of the substrate particles when too short or too rough, ineffective coatings when too far; (2) the actual contact of many of the falling particles with the hot spray heads leads to melted substrate, which drips onto the substrate bed, causing serious agglomeration of some of the substrate; (3) visual monitoring of the individual spray operation is impossible; and (4) dusting is serious when the entire section of the rotating drum is filled with falling particles, dust formed by attrition, and solidified spray mist, all of which can be carried from the system by the cooling or heating air flowing through the coating unit. This increases antipollution equipment requirements.
The coating procedure as practiced by Blouin et al (U.S. Pat. No. 3,295,950, Jan. 3, 1967; U.S. Pat. No. 3,342,577, Sept. 19, 1967) and by Shirley et al (patent application Ser. No. 346,711, filed Mar. 30, 1973 assigned to the assignee of the instant application and abandoned in favor of application Ser. No. 449,322, filed Mar. 8, 1974, now U.S. Letters Pat. No. 3,903,333, issued Sept. 2, 1975), are almost identical in nature, i.e., the directing of the atomized coating material only onto the rolling bed of substrate in a horizontal rotary drum having a relatively smooth interior (no lifting vanes or flights, see U.S. Pat. No. 2,741,545, Nielsson, Apr. 10, 1956). The latter art, i.e., Shirley, discloses certain improvements in the former which, according to the example data disclosed, does result in somewhat more uniform coatings than those of the former art.
However, both practice essentially the same approach as described above and, therefore, both suffer from the same serious disadvantage that precludes a truly homogeneous moving bed of particles of different sizes and therefore true uniformity of the coating, namely, the segregation by particle size of particles of varying sizes (always present in fertilizer production, for example) that occurs in a smooth, horizontal rotary drum. This segregation, or demixing, is well documented in the extensive work of Campbell et al [Chemical Engineering 73 (19), 179-185 (Sept. 12, 1966)] and McDonald et al [British Chemical Engineering 7 (10), 749-753 (Oct. 1962)--Part I; ibid 7 (11), 823-27 (Nov. 1962)--Part II; and ibid 7 (12), 922-23 (Dec. 1962)--Part III]. Although the degree of demixing, or segregation, that occurs in a smooth drum may be reduced by proper choice of operating and equipment variables such as bed depth, or degree of drum loading, drum speed (expressed as percent of critical speed, the critical speed, i.e., revolutions/minute, of a smooth drum being defined as 76.5/.sqroot.D, where D = drum diameter in feet), and ratio of drum diameter to drum length, it cannot be eliminated. As a result, smaller particles tend to segregate from the larger particles by going to the point of lowest particle velocity, namely, the center of the cross sectional area of the bed and pass on through the drum without coming to the surface of the bed. This, of course, prevents these particles from being coated by the liquid spray.
It is now believed that the present invention as described in succeeding sections of this application will overcome the serious disadvantages encountered in the practice of prior art previously discussed.
It is therefore the primary object of the present invention to provide essentially random motion of particles with respect to each other and therefore a truly homogeneous moving mass of particles in a simple horizontal rotary drum so that in any otherwise simple liquid spray coating process, all of the particles will be equally exposed to the liquid spray material as they pass through the drum regardless of range of size or shape of the particles, thus providing a means of producing highly uniform coatings on the particles.
It is a further object of the present invention to provide the means of applying highly uniform coatings to particulate matter by spray coating with a liquid media in a simple, widely used type of unit that has a high-tonnage production capacity per unit of cost of the equipment.